Commercial flat and low-sloped roofing systems provide moisture resistance, thermal resistance (R-value) and dimensional stability as part of the building envelope.
Flat and low-slope roof membranes fall into two main materials categories a) polymer based and b) bitumen based. Within polymer based low-slope roof systems there are two major types: Thermosets (TS), including Ethylene Propylene Diene Monomer (EPDM) and Chlorosulfonated Polyethylene (CSPE), and Thermoplastics (TP), including Poly Vinyl Chloride (PVC), Thermoplastic Polyolefin (TPO), Chlorinated Polyethylene (CPE) and Keytone Ethylene Ester (KEE). Within the bitumen based low-slope roof systems there are two categories: Built-up Roofing (BUR) including Asphalt and Coal Tar and Modified Bitumen (Mod. Bit.) including Atactic polypropylene (APP) and Styrene-Butadiene Styrene (SBS).
Membrane roof materials and systems are designed to meet the requirements of the building in specific climatic conditions and are specified based on the cost, long-term weatherability, resistance to stress caused by expansion and contraction from fluctuations in temperature, ultraviolet light resistance, solar reflectance and emittance, tensile strength, water and fire resistance, wind uplift, elongation and thermal expansion, dynamic puncture resistance and resistance to rooftop contaminants such as acid rain and air pollution. Exposure to extreme environments, ultraviolet rays and thermal stresses age the useful life of roof membrane systems.
Roof membrane systems are either mechanically fastened, ballasted, heat welded or fully adhered with adhesives and solvents. Membranes are both un-reinforced and reinforced with polyesters or fiberglass for strength and dimensional stability and available in a range of thickness from 45 mils to 90 mils. In the roofing industry, thicker roof membranes are considered more durable.
Flexible roof membranes are attached to the roof using one of three methods. Ballasted roof membranes require that the membrane material be laid directly over roof insulation or the roof deck and attached at the perimeter and held in place by gravel ballast or pavers. This system offers a low installation cost. However, the system is restricted by the weight that the roof deck is designed to support. In addition, the ballast material must be removed to locate leaks, making repairs time consuming and costly. In a second method, fully-adhered roof membranes require that the roof membrane be adhered to the roof with contact adhesive. This lightweight system yields high wind resistance and can be used with most deck types. However, fully adhered roof systems depend on the roof insulation to which they are adhered for wind uplift resistance. Roof pads are also often required in high traffic areas to prevent the compression of insulation, delamination of insulation facers, and general damage to the membrane, such as punctures and tears. In a third method, mechanically-attached roof membranes are attached to steel and wood decks with fasteners.
The Environmental Protection Agency's ENERGY STAR® Roof Products Program has established a minimum standard that requires low-slope reflective roof products to have an initial solar reflectance of at least 65 percent, and a reflectance of at least 50 percent after three years of weathering to be considered a ‘Cool Roof’, energy efficient or high performance roof. Cool Roofs typically incorporate bright white membranes that keep moisture out while reflecting ultraviolet and infrared radiation, protecting the underlying insulation and roofing substrate from deterioration. These Cool Roof systems reduce building energy consumption by up to 40 percent, improve insulation performance to reduce winter heat loss and summer heat gain and can potentially reduce HVAC equipment capacity requirements. The Cool Roof reflects light and heat away from the roof deck to assist with maintaining low air conditioning loads and considered an energy efficiency measure. Reflecting light off the roof membrane results in lower lifetime membrane temperatures and lengthen the life of the roofing system. The success of sustainability initiatives such as the U.S. Green Building Council's LEED rating system, have encouraged the roofing industry to develop cool roof systems that meet or exceed requirements for the U.S. EPA's ENERGY STAR® label for roofing membranes.
The term “photovoltaic” is derived from the root words “photo”, meaning light, and “voltaic”, meaning electricity. Sunlight, the common power source for photovoltaic systems, is composed of photons. The amount of energy in a photon is proportional to the frequency of its light. When photons strike a photovoltaic cell, the photons are either reflected or absorbed. When a photon is absorbed, its energy is transferred to an atom of the cell, where an electron leaves its normal position associated with that atom and moves into a current. A portion of the energy created is electrical, while another portion is thermal in nature.
Photovoltaic cells react to different wavelengths of light as a function of their material composition. Common photovoltaic cell materials include: single crystalline silicon, polycrystalline silicon and amorphous silicon, gallium arsenide, copper indium diselenide, cadmium telluride, dye-sensitive and nano-technologies. In addition, photovoltaic cells, laminates and modules can be composed of two or more layers of different photovoltaic materials with different wavelengths and bandwidth sensitivities to yield improved energy conversion efficiencies.
When exposed to light, photovoltaic cells increase in temperature, which affects each photovoltaic cell materials' energy conversion efficiency in a unique manner. This is measured and known as the Installed Nominal Operating Cell Temperature (INOCT). For example, the efficiency of the crystalline silicon solar cell strongly depends on its operating temperature and the efficiency of the amorphous is less affected by its operating temperature. Accordingly, thin film and flexible amorphous silicon systems have been commercially accepted and flush mounted to membrane roof systems. U.S. Pat. No. 4,860,509 and U.S. Patent Publication No. 2005/0072456 teach examples of flexible, photovoltaic material roofing assemblies, adhered to a single-ply roofing membrane. In the field, however, flexible amorphous silicon cell temperatures have been documented to exceed 77° C. (170° F.). Canadian Patent No. 2,554,494 provides an example of the use of crystalline photovoltaic cells, in a layered fashion that includes a base, flexible membrane layer, a semi-rigid support layer, the photovoltaic layer and a protective layer forming a unitary structure to be adhered directly to the roof. Each of these photovoltaic membrane systems, however, allows the transmission of heat from the photovoltaic cells to the building structure, limiting the operative efficiency and life of the photovoltaic cells and damaging the structural materials of the building and its protective envelope system.
In the field, it is known in the photovoltaic community that for each degree Celsius that a crystalline photovoltaic cell increases over its standard test conditions (STC) rated temperature, its performance goes down by 0.05% of its rated power. Additionally, when photovoltaic cells are integrated into an insulated roof system, there is little opportunity for heat loss off the backside of the modules and this heat is transferred into the building envelope.
Most crystalline silicon based PV arrays exhibit a relative efficiency temperature sensitivity of 0.5%/1° C. It is estimated that thin film amorphous silicon and cadmium arrays, although not as well documented due to their newness in the field, exhibit less than half of the performance temperature sensitivity of crystalline photovoltaic arrays. SANDIA National laboratory conducted a study that states that, “maintaining an open rack air flow results in 20° C. reduction in average operating temperature, a nearly a 10% greater amount of annual energy (for crystalline silicon), and an untold increase in life expectancy compared to direct mounted arrays on an insulated roof surface.” Unfortunately, photovoltaic specialists have focused on the photovoltaic's INOCT and have not addressed the architectural impact of the increase of cell temperature on the roof system beneath, the heat transfer impact on the buildings thermal performance or the integrity of the building envelope.
Since the late 1980's, building integrated photovoltaic (BIPV) technology and systems have been developed as part of a movement towards whole building design and the efficient, sustainable use of resources. The objective of BIPV technology is to have one system that serves as the protective building envelope and also generates electric power for use within the building in the form of electric roof membranes, electric windows and glazing, electric awnings, electric roof tiles, electric standing seam metal roofing and the like. U.S. Pat. No. 6,553,729 and U.S. Pat. No. 6,729,081 teach examples of photovoltaic modules that are adhered directly to a roof, wall or other portion of the building structure using an adhesive. These photovoltaic systems generate on-site distributed electric power that will offset building electrical loads, decrease building electrical demand, put less demand stress on the local utility transmission system, allow surplus power to be fed back into the utility grid and may provide continuous power supply during utility grid outage.
Photovoltaic membrane roof systems installed on low-sloped roofs may be attached to the roof using mechanical fasteners, ballast or adhesives. As the photovoltaic cell heats up, thermal energy is trapped behind its surface, against the roof membrane, insulation board and deck beneath the photovoltaic cell. Over time, the photovoltaic system effectively stresses and ages the building system underneath establishing a core physical incompatibility of a direct interface between the two systems. Accordingly, prior art systems that directly attach photovoltaic systems to roof decks tend to reduce the performance life of the building materials by elevating temperatures in the building envelope system. Elevated temperatures accelerate and increase the degradation rates of most materials. A common rule of thumb for polymers states that the material life expectancy is reduced by half for each 10° C. rise in average temperature.
Photovoltaic systems mounted directly onto the building envelope trap heat into the roof deck creating a series of hot spots or heat islands on the roof which not only stresses and accelerate the aging of the roof membrane and deck underneath but negatively affecting the building's energy system. The trapped thermal energy can result in greater heat transfer to the building interior and produce a greater demand for air conditioning, which results in a strain on both operating costs and the electric power grid. Such systems further inhibit the ability of the roof insulation to work optimally, in effect requiring that air conditioning loads increase, due to the photovoltaic system. This is inconsistent with the objective of using the photovoltaic system.